2.2 LBBAP procedural technique
All procedures, including conventional RVP and LBBAP, were performed by operators without previous LBBAP experience. LBBAP was performed with a 5.6Fr SDL with an extendable helix (Solia S60, Biotronik SE & Co KG, Berlin, Germany) delivered through a pre-shaped sheath (Selectra 3D, Biotronik). The lead was prepared as described previously by De Pooter et al.8 Briefly, it was prepared by exposing the extendable screw by turning the outer pin 10 to 12 times clockwise, followed by five additional clockwise turns of the outer pin using the standard stylet guide tool delivered with the lead to avoid partial unwinding of the extendable helix (Supplementary Figure S1). We positioned a combined His/right ventricle (RV) catheter for ventricular backup pacing and His potential mapping as a landmark. The lead tip was placed at 1.5-2 cm toward the RV apex from the His area in the right anterior oblique view and perpendicular to the septum in the left anterior oblique view, as described in previous studies (Supplementary Figure S2 a).3, 9 If His potential was not visible, we used the method similar to the simplified nine-partition method, which was also introduced in a previous study (Supplementary Figure S2 b).10 Surface ECGs (12-lead) and intracardiac electrograms were continuously monitored using an electrophysiology recording system. Paced QRS morphology and unipolar pacing impedance were monitored. Left bundle branch (LBB) capture was confirmed using published criteria.3, 11 Before lead penetration, the stylet was fully advanced to the pacing lead tip. For lead penetration, we advanced the pacing lead by fast rotation of the whole lead body 5-10 times to overcome septal resistance. Subsequently, pacing V1 morphology was checked whenever it was rotated one or two times.8When pacing the right side of the interventricular septum initially, the QRS of V1 showed the LBB block (LBBB) pattern. As the pacing lead penetrated the septum, the QRS of V1 gradually changed to the right bundle branch block (RBBB) pattern, and the pacing site was judged to be the LBB. At this point, a fast peak left ventricular activation time (LVAT) in leads V5 to V6 of approximately 75-80 ms was noted, which reflects the LBBAP. The LBB potential was recorded when the pacing lead was near or at the LBB.